Ribosome-inactivating proteins (RIPs) are plant toxins that were identified for their ability to irreversibly damage ribosomes, thereby causing arrest of protein synthesis and induction of cell death. The RIPs purified from Adenia plants are the most potent ones. Here, we describe a novel toxic lectin from Adenia heterophylla caudex, which has been named heterophyllin. Heterophyllin shows the enzymatic and lectin properties of type 2 RIPs. Interestingly, in immunoreactivity experiments, heterophyllin poorly cross-reacts with sera against all other tested RIPs. The cytotoxic effects and death pathways triggered by heterophyllin were investigated in three human-derived cell lines: NB100, T24, and MCF7, and compared to ricin, the most known and studied type 2 RIP. Heterophyllin was able to completely abolish cell viability at nM concentration. A strong induction of apoptosis, but not necrosis, and the involvement of oxidative stress and necroptosis were observed in all the tested cell lines. Therefore, the enzymatic, immunological, and biological activities of heterophyllin make it an interesting molecule, worthy of further in-depth analysis to verify its possible pharmacological application.

Ricin, a type 2 ribosome-inactivating protein, is a lethal toxin found in castor bean seeds. Although the systemic toxicity of ricin has been extensively studied, its localized effect on the gastrointestinal tract remains a critical concern, particularly in the case of oral ingestion. This study investigates the cytotoxic effects of ricin on human intestinal epithelial cell lines and its impact on epithelial barrier integrity. Ricin cytotoxicity was assessed on the intestinal-derived HT29 and Caco-2 cell lines using dose– and time–response assays, while the epithelial integrity was evaluated via Trans-Epithelial Electrical Resistance (TEER) measurements in Caco-2 monolayers. Cell death was determined through flow cytometry analysis, and the protective effects of cell death inhibitors and antioxidant scavengers were investigated on ricin-intoxicated cells. Ricin showed high cytotoxicity on HT29 and Caco-2 cells, with EC₅₀ values in the nM range after 24–72 h of intoxication. Moreover, ricin strongly reduced TEER values in Caco-2 cells at 0.1–1 nM after 24 h of treatment. At a 1 nM concentration, ricin cytotoxicity can be significantly prevented by pre-incubating cells with the cell death inhibitors Z-VAD or necrostatin-1 and the antioxidant scavenger catalase, butylated hydroxyanisole or sodium pyruvate, demonstrating the involvement of apoptosis/necroptosis and oxidative stress in ricin cell death pathways and mechanisms.

Saporin is a type 1 ribosome-inactivating protein widely used as toxic payload in the construction of targeted toxins, chimeric molecules formed by a toxic portion linked to a carrier moiety. Among the most used carriers, there are large molecules (mainly antibodies) and small molecules (such as neurotransmitters, growth factors and peptides). Some saporin-containing targeted toxins have been used for the experimental treatment of several diseases, giving very promising results. In this context, one of the reasons for the successful use of saporin lies in its resistance to proteolytic enzymes and to conjugation procedures. In this paper, we evaluated the influence of derivatization on saporin using three heterobifunctional reagents, namely 2-iminothiolane (2-IT), N-succinimidyl 3-(2-pyridyldithio)propionate (SPDP) and 4-succinimidyloxycarbonyl-α-methyl-α-[2-pyridyldithio]toluene (SMPT). In order to obtain the highest number of inserted -SH groups with the lowest reduction of saporin biological activities, we assessed the residual ability of saporin to inhibit protein synthesis, to depurinate DNA and to induce cytotoxicity after derivatization. Our results demonstrate that saporin maintains an excellent resistance to derivatization processes, especially with SPDP, and permit us to define reaction conditions, in which saporin biological properties may not be altered. Therefore, these findings provide useful information for the construction of saporin-based targeted toxins, especially with small carriers.

The most successful alternative to traditional surgery for ocular muscle spasm treatment is the intramuscular injection of botulinum toxin (BTX), which allows the maintenance of the muscle dynamics and the absence of scars. However, the main BTX disadvantage is its nonpermanent effect. A possible way for overcoming this obstacle could be represented by the enzymatic surgery using plant toxins known as ribosome-inactivating proteins (RIPs). In this paper, two highly toxic RIPs, namely, ricin and stenodactylin, were considered in a preliminary study for their possible use in the treatment of strabismus and oculofacial dystonias, as alternatives to BTX. Both RIPs showed a strong cytotoxic effect against rhabdomyosarcoma cell lines and myotube differentiated cells, with stenodactylin being about 10-fold more toxic than ricin. Moreover, stenodactylin showed a much higher cytotoxicity on myoblasts than on rhabdomyosarcoma cells. In our experimental conditions, stenodactylin did not damage conjunctival cells. Despite the limitations due to in vitro experiments, our data show that the high cytotoxicity of stenodactylin allows the use of a very low dose and, consequently, of very low injection volumes. This can represent a great advantage in the case of in vivo locoregional treatment. Furthermore, it is possible to modulate the chemoablation of myocytes while destroying myoblasts, thus reducing regenerative phenomena. The risk of cytotoxicity to surrounding tissues would be strongly reduced by the low injected volume and the relative resistance of conjunctival cells. In conclusion, our data suggest that stenodactylin and ricin could represent potential candidates to substitute BTX in ocular dystonia therapy.

Background/Objectives: Ribosome-inactivating proteins (RIPs) are plant-derived enzymes with potent cytotoxic activity, widely studied as anticancer agents, particularly as toxic payloads in immunoconjugates. Despite numerous encouraging results reported, their clinical application has been limited by their immunogenicity. RIPs from edible plants have been proposed as potentially more suitable candidates due to their possible improved tolerability. However, this aspect still requires validation in vivo in animal models. This study investigated the cytotoxic activity, mechanisms of action and translational potential of sodin 5 (a recently characterized type 1 RIP derived from the edible plant Salsola soda L.) in human colon cancer models, comparing it to the well-known type 1 RIP saporin-S6. Methods: The effects of sodin 5 and saporin-S6 on cell viability, cell death mechanisms and epithelial barrier integrity were assessed on HT29 and Caco-2 cell lines. Sodin 5 cross-reactivity with other anti-type 1 RIP sera was evaluated by ELISA. Finally, its structural characteristics were analyzed. Results: Sodin 5 showed a cytotoxic effect comparable to that of saporin-S6 in HT29 and Caco-2 colon cancer cells, with time- and concentration-dependent reductions in viability. Both type 1 RIPs disrupted the integrity of the intestinal epithelial barrier in mono- and co-culture models and predominantly activated the apoptotic pathway, without inducing necrosis. Sodin 5 exhibited limited immunological cross-reactivity and a conserved catalytic core, supporting its potential relevance as a therapeutic payload for intestinal cancer therapy. Conclusions: Our results indicate that sodin 5 possesses promising characteristics for anticancer applications, particularly in the treatment of intestinal malignancies, where local exposure and repeated administration are often required.

Indicitin is an atypical ribosome-targeting enzyme isolated from the edible mushroom Calocybe indica , structurally classified as thaumatin-like protein. This toxin has low toxicity toward several cell types and is non-harmful to the gut microbiota.